The present invention relates to a loudspeaker system and particularly to a loudspeaker system for providing an expanded soundstage and three dimensional effect within a given listening area.
In simplest terms, a stereophonic reproduction system employs plural microphones to pick up sounds emanating from various areas on a stage, and corresponding transducers or loudspeakers are driven separately to reproduce the sounds received at the various microphones. Of course, signals may be manipulated to varying degrees and a number of channels may be combined to provide essentially two output channels which are recorded for reproduction in a customary stereophonic loudspeaker system. The concept of providing realistic sound in this manner is valid in many respects because a person with normal hearing picks up sounds binaurally. However, speaker design, speaker location, room acoustics, and electronic circuitry are all critical factors in achieving the desired results. If speakers are placed farther apart to give the perception of a larger soundstage, a "hole" may become apparent between the two speakers, and sound will appear to emanate from the right and/or left loudspeaker. In addition adjacent walls will cause unwanted reaction to the sound waves and will tend to interfere with desired wave radiation patterns. On the other hand, if the speakers are placed too close together, the middle information will dominate and the soundstage will appear much narrower. Also, movement of the listener from the apex of an equal sided triangle formed by the speakers and himself will cause a perceived shift in the program material from one side of the soundstage to the other, i.e. deteriorating the stereophonic effect.
If two loudspeakers are fed "out of phase" with the same signal it is possible to achieve the illusion of sound originating to the left of both speakers or to the right of both speakers depending upon which speaker is out of phase from normal. The effect is not a particularly natural one. It would be desirable to provide a loudspeaker system presenting the illusion of sound emanating from an entire soundstage having dimensions beyond the distance between two loudspeakers, such as speakers 12 and 14 in FIG. 1, e.g. along the stage 10 between points 18 and 20 in FIG. 1.
One prior art approach to providing a broadened soundstage is illustrated in FIG. 2 wherein speaker 12 in FIG. 1 is replaced by a pair of speakers 26, 32 and speaker 14 is replaced by a pair of speakers 28, 30. The principal speakers 26 and 28 are driven from conventional stereo amplifier channels 22 and 24, but auxiliary or enhancement speakers 30 and 32 are driven in a reversed phase sense from the opposite channel. Enough wavefront subtraction is produced so that sound will appear to originate to the left and to the right of the group of speakers as well as therebetween, if the speakers aren't too far apart and if the auxiliary speakers 30 and 32 are operated at an amplitude level less than that of the main speakers 26 and 28. Unfortunately, the FIG. 2 approach has certain acoustical drawbacks including major shifts in perceived locations as well as cancellations of image, apparently due to phase anomalies which occur whenever the listener moves from a given spot, e.g. away from apex location 16 in FIG. 1. This phenomenon apparently takes place primarily because the extra speakers 30 and 32 have different points of radiation from those of the main speakers, even if located in the same enclosure or cabinet with the main speakers. Thus the ear, which is phase sensitive, picks up the time difference in the sound wave radiation pattern from each enclosure and renders the expanded soundstage system "believable" only at certain positions.
It is also possible to perform the function of the FIG. 2 system electronically, wherein single transducers or loudspeakers are substituted for the pairs 26, 32 and 38, 30. Thus, the lefthand speaker in a stereophonic system is driven from an electronic adder or summing point which receives both the output from the lefthand amplifier channel 22 and a reversed and attenuated output from the righthand channel 24. Unfortunately the subtraction or summation achieved is frequently not natural enough to provide a sound which the listener will perceive as coming from a broadened soundstage.
In spite of the various problems, the prior art approaches do give the listener some sensation of an expanded soundstage, and the systems are interesting and viable. They do suffer from lack of realism, particularly if the listener moves from the central location or apex indicated at 16 in FIG. 1.
Experiments using the technique of FIG. 2 have led to the conclusion that optimum results could be obtained if the two drivers, such as 26, 32, could occupy the same physical space in an enclosure and be of identical design and construction. This is an apparent impossibility.